Method of reducing the quantity of residual monomers in aqueous dispersions of polymers

ABSTRACT

A process for reducing the amount of residual monomer in aqueous polymer dispersions operates by aftertreatment with an initiator system essentially comprising 
     a) from 0.001 to 5% by weight, based on the total monomer amount used to prepare the polymer dispersion, 
     a 1 ) of an oxidizing agent 
     
       
         R 1 OOH, 
       
     
      where R 1  is hydrogen or a C 1 -C 8 -alkyl or a C 6 -C 12 -aryl group, and/or 
     a 2 ) of a compound which in aqueous medium releases hydrogen peroxide, and 
     b) from 0.005 to 5% by weight, based on the total monomer amount used to prepare the polymer dispersion, 
     b 1 ) of an α-hydroxycarbonyl compound                    
      where R 2  and R 3  independently of one another are hydrogen and/or a C 1 -C 12 -alkyl group which may contain functional groups and/or may be olefinically unsaturated, or R 2  and R 3  optionally, by way of methylene groups, form a ring structure which may contain functional groups and/or may be olefinically unsaturated, and/or 
     b 2 ) of a compound which in aqueous medium releases such α-hydroxycarbonyl compounds, and 
     c) advantageously, catalytic amounts of a polyvalent metal ion which is able to exist in a plurality of valence states.

The invention relates to a process for reducing the amount of residualmonomer in aqueous polymer dispersions by chemical aftertreatment with aspecific redox initiator system.

Following their preparation by free-radical polymerization orcopolymerization, aqueous polymer dispersions include not only a polymersolids fraction of from 30 to 75% by weight but also, owing to theincomplete polymerization of the monomers employed in the free-radicalmain polymerization, which is usually carried out to a monomerconversion of 95 and preferably 98 to 99% by weight, an unwantedfraction of unpolymerized free monomers (residual monomers). On mainlytoxicological grounds the market Far requires aqueous polymer systemshaving a low residual monomer content with no change in processing anduse properties.

In addition to the nonchemical methods such as stripping with inert gasor steam, a wide variety of chemical methods, such as described, forexample, in EP-B 003 957, EP-B 028 348, EP-B 563 726, EP-A 764 699, U.S.Pat. No. 4,529,753, DE-A 37 18 520, DE-A 38 34 734, DE-A 42 32 194 andDE-A 195 29 599, is available for lowering residual monomer contents ofaqueous polymer dispersions.

For the use of carbonyl compounds and/or their reaction products in theaftertreatment of aqueous polymer dispersions it is necessary to startfrom the following prior art.

According to WO 95/33775, aqueous polymer dispersions can beaftertreated using redox systems whose reducing agent comprises anadduct of hydrogen sulfite anion and a ketone of 3 to 8 carbon atoms,and/or the conjugate acid of said adduct. Aftertreatment is performed inthe presence of metal compounds that are soluble in the aqueous medium.

For the reduction of residual monomer contents, EP-A 767 180 advises aredox initiator system comprising organic hydroperoxides whosesolubility in water is, at best, poor and, inter alia, adducts ofaldehydes having a carbon chain of 4 to 6 carbon atoms with bisulfites.

The German patent application with the file reference 197 411 87.8,unpublished at the priority date of the present application, disclosesusing a system comprising an oxidizing agent and an organic α-hydroxycarboxylic acid for chemical removal of residual monomers.

In the German patent application with the file reference 198 391 99.4,likewise unpublished at the priority date of the present application,the use of oxidizing agents in combination with a redox systemconsisting of an aldehyde and an inorganic dithionite is disclosed forbringing about depletion of residual monomers.

It is an object of the present invention to provide a new and effectiveprocess for reducing the amount of residual monomer in aqueous polymerdispersions. The intention is also that the reducing of the amount ofresidual monomer should be easy to utilize industrially without theformation of microcoagulum.

We have found that this object is achieved and thus that the amount ofresidual monomers in aqueous polymer dispersions can be effectivelyreduced if the aftertreatment of the aqueous polymer dispersionscomprising residual monomers is accompanied by the addition of a redoxinitiator system comprising essentially

a) from 0.001 to 5% by weight, based on the total monomer amount used toprepare the polymer dispersion,

a₁) of an oxidizing agent

R¹OOH,

 where R¹ is hydrogen or a C₁-C₈-alkyl or a C₆-C₁₂-aryl group, and/or

a₂) of a compound which in aqueous medium releases hydrogen peroxide,and

b) from 0.005 to 5% by weight, based on the total monomer amount used toprepare the polymer dispersion,

b₁) of an α-hydroxycarbonyl compound

 where R² and R³ independently of one another are hydrogen and/or aC₁-C₁₂-alkyl group which may contain functional groups and/or may beolefinically unsaturated, or R² and R³ optionally, by way of methylenegroups, form a ring structure which may contain functional groups and/ormay be olefinically unsaturated, and/or

b₂) of a compound which in aqueous medium releases suchα-hydroxycarbonyl compounds, and

c) advantageously, catalytic amounts of a polyvalent metal ion which isable to exist in a plurality of valence states.

The oxidizing agent of the redox initiator system should be in aposition to form free radicals. Oxidizing agents employed in the redoxsystem are preferably hydrogen peroxide but also include sodiumperoxide, potassium peroxide, sodium perborate, and other precursorswhich in aqueous medium form hydrogen peroxide. It is also possible, forexample, to employ ammonium, potassium or sodium persulfate,peroxodisulfuric acid and its salts, ammonium, potassium or sodiumperphosphate or diperphosphate, potassium permanganate, and other saltsof peracids. Also suitable in principle are organic hydroperoxides, suchas tert-butyl hydroperoxide and cumene hydroperoxide. It is, however,also possible to employ mixtures of different oxidizing agents mentionedabove.

The amount of added oxidizing agent is usually within a range from 0.001to 5, preferably from 0.002 to 3, with particular preference from 0.003to 2, with very particular preference from 0.01 to 1.5 and, preferably,from 0.02 to 1% by weight, based on the total monomer amount.

Suitable reducing agents are generally aliphatic α-hydroxycarbonylcompounds, such as aliphatic α-hydroxy aldehydes and/or aliphaticα-hydroxy ketones, isomeric compounds thereof and/or functional groupsubstituted compounds thereof and/or olefinically unsaturated compoundsthereof and mixtures thereof, and also precursors which in aqueoussolution release these α-hydroxycarbonyl compounds, examples beingacetals and mercaptals. Examples that may be mentioned ofα-hydroxycarbonyl compounds are glycol aldehyde and/or its dimer2,5-dihydroxy-1,4-dioxane, phenyl glycol aldehyde,2-hydroxy-3-phenylpropionaldehyde, glyceraldehyde and its higherhomologous compounds, such as aldotetroses, aldopentoses andaldohexoses, and also α-hydroxyacetone, α,α′-dihydroxyacetone,1-hydroxy-2-butanone, 1-hydroxy-2-pentanone, 1-hydroxy-2-hexanone,3-hydroxy-2-butanone (acetoin), 4-hydroxy-2-hepten-5-one,2-hydroxy-3-pentanone, 3-hydroxy-2-pentanone, 3-hydroxy-4-heptanone,4-hydroxy-3-heptanone, 4-hydroxy-2,2-dimethyl-3-pentanone,3-hydroxy-2,2-dimethyl-4-pentanone, 2-hydroxy-1-phenyl-1-propanone,1-hydroxy-1-phenyl-2-propanone, 1-hydroxy-1-phenyl-2-butanone,2-hydroxy-1-phenyl-1-butanone, 2-hydroxy-1,2-diphenylethanone (benzoin),2-hydroxy-1-phenyl-1,4-pentanedione,1-hydroxy-1-phenyl-2,4-pentanedione, and cyclic α-hydroxy ketones, suchas 2-hydroxycyclohexanone (adipoin) and 2-hydroxycyclopentanone(glutaroin). Preference is given to the use of α-hydroxyacetone,α,α′-dihydroxyacetone, 1-hydroxy-2-butanone, 1-hydroxy-2-pentanoneand/or 3-hydroxy-2-butanone (acetoin), with particular preference to theuse of α-hydroxyacetone and/or α,α′-dihydroxyacetone.

The amount of added reducing agent is customarily within the range from0.005 to 5, preferably from 0.01 to 3, with particular preference from0.03 to 2 and, with very particular preference, from 0.05 to 1% byweight, based on the total monomer amount. Higher amounts of reducingagent, although possible, are not generally sensible from the economicstandpoint.

The metal compounds advantageous for the aftertreatment are,customarily, completely soluble in the aqueous medium of the polymerdispersion, and their metallic component, moreover, is capable ofexisting in a plurality of valence states. The dissolved metal ions havea catalytic effect and assist the electron transfer reactions betweenthe actually active oxidizing and reducing agents. Suitable dissolvedmetal ions are principally iron, copper, manganese, vanadium, nickel,cobalt, titanium, cerium, and chromium ions. It is of course alsopossible to use mixtures of different, mutually compatible metal ions,such as the system Fe^(2/3+)/VSO₄ ⁻. Preferably, iron ions are employed.

The dissolved metal ions are used in catalytic amounts based on the massof total monomer, customarily from 1 to 1000, preferably from 5 to 500and, with particular preference, from 10 to 100 ppm.

For the aftertreatment of the polymer dispersion which is heated at fromabout 50 to 130, preferably from 60 to 120 and, with particularpreference, from 80 to 100° C. the components of the initiator systemused in accordance with the invention are judiciously metered ingradually with stirring, preferably under atmospheric pressure or,alternatively, under a pressure of greater than or less than I bar(absolute), metered addition taking place simultaneously or insuccession, in the latter case preferably with addition of the oxidizingagent first. A particularly favorable procedure is the simultaneousmetered addition of oxidizing and reducing agent by way of two separatefeed streams. In this case the initiator components, for example, can beadded from above, from below or through the side of the reactor.Preferably, however, the initiator system is metered in from below.Since the optimum duration of initiator metering is dependent on themonomer composition, on the size of the reaction mixture, and on thereactor geometry, inter alia, it is judicious to determine said durationin preliminary experiments. Depending on the task at hand, the durationof addition of initiator may extend from a few seconds to several hours.It is particularly favorable for the metal compound, which is employedin catalytic amounts, to be added to the polymer dispersion prior to theaddition of the oxidizing and reducing agent.

The aftertreatment is generally conducted at a pH of ≦10. To adjust thepH of the polymer dispersion it is possible in principle to use bases,such as sodium hydroxide solution, aqueous ammonia, or triethanolamine,for example. For the aftertreatment of the polymer dispersion afavorable pH range is that ≧2 and ≦10, with preference being given to apH range of between >6 and ≦8. If the pH is established using bases,however, the catalyzing metal ions could be converted into poorlysoluble hydroxides or hydroxo complexes. To ensure sufficientconcentrations of metal ions during the aftertreatment, therefore, it isparticularly advantageous to add complexing agents, such asethylenediaminetetraacetic acid, nitrilotriacetic acid anddiethylenetriaminepentaacetic acid and/or the respective sodium salts,and/or to use stable metal ion complexes, such as iron (III)/sodiumethylenediaminetetraacetate, for example.

The process of the invention is particularly suitable for reducing theamount of residual monomer in aqueous polymer dispersions obtainable byfree-radical emulsion polymerization of monomers having at least oneethylenically unsaturated group.

Suitable monoethylenically unsaturated monomers for the process of theinvention include, in particular, monomers which can be subjected tofree-radical polymerization in a simple manner, examples being theolefins ethylene, vinylaromatic monomers such as styrene,o-methylstyrene, o-chlorostyrene or vinyltoluenes, esters of vinylalcohol and monocarboxylic acids of 1 to 18 carbon atoms, such as vinylacetate, vinyl propionate, vinyl n-butyrate, vinyl laurate and vinylstearate, esters of preferably C₃-C₆ α,β-monoethylenically unsaturatedmono- and dicarboxylic acids, such as especially acrylic, methacrylic,maleic, fumaric and itaconic acid, with generally C₁-C₁₂-, preferablyC₁-C₈- and in particular C₁-C₄-alkanols, such as, in particular, methyl,ethyl, n-butyl, isobutyl and 2-ethylhexyl acrylate and methacrylate,dimethyl or di-n-butyl maleate, nitriles of α,β-monoethylenicallyunsaturated carboxylic acids, such as acrylonitrile, and C₄₋₈ conjugateddienes, such as 1,3-butadiene and isoprene. In the case of aqueouspolymer dispersions produced exclusively by the method of free-radicalaqueous emulsion polymerization, said monomers generally form theprincipal monomers which, based on the total amount of monomers to bepolymerized by the process of free-radical aqueous emulsionpolymerization, normally account for a proportion of more than 50% byweight. As a general rule, the solubility of these monomers in waterunder standard conditions (25° C., 1 atm) is moderate to poor.

Examples of monomers having a heightened solubility in water under theabove conditions are α,β-monoethylenically unsaturated monocarboxylicand dicarboxylic acids and their amides, examples being acrylic acid,methacrylic acid, maleic acid, fumaric acid, itaconic acid, acrylamide,and methacrylamide, and also vinylsulfonic acid and its water-solublesalts, and N-vinyl-pyrrolidone.

In the case of aqueous polymer dispersions produced exclusively by themethod of free-radical aqueous emulsion polymerization, theabovementioned monomers of heightened solubility in water are normallycopolymerized only as modifying monomers, in amounts, based on the totalamount of the monomers to be polymerized, of less than 50% by weight,generally from 0.5 to 20% by weight and, preferably, from 1 to 10% byweight.

Monomers which usually enhance the internal strength of the films of theaqueous polymer dispersions normally have at least one epoxy, hydroxyl,N-methylol or carbonyl group, or at least two nonconjugatedethylenically unsaturated double bonds. Examples thereof areN-alkylolamides of C₃-C₁₀ α,β-monoethylenically unsaturated carboxylicacids, among which very particular preference is given toN-methylolacrylamide and N-methylolmethacrylamide, and their esters withC₁-C₄-alkanols. Also suitable are monomers having two vinyl radicals,two vinylidene radicals, or two alkenyl radicals. Particularlyadvantageous in this context are the diesters of dihydric alcohols withα,β-monoethylenically unsaturated monocarboxylic acids, among whichacrylic and methacrylic acid are preferred. Examples of such monomershaving two nonconjugated ethylenically unsaturated double bonds arealkylene glycol diacrylates and dimethacrylates such as ethylene glycoldiacrylate, 1,2-propylene glycol diacrylate, 1,3-propylene glycoldiacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycoldiacrylates and ethylene glycol dimethacrylate, 1,2-propylene glycoldimethacrylate, 1,3-propylene glycol dimethacrylate, 1,3-butylene glycoldimethacrylate, 1,4-butylene glycol dimethacrylate, and alsodivinylbenzene, vinyl methacrylate, vinyl acrylate, allyl methacrylate,allyl acrylate, diallyl maleate, diallyl fumarate,methylenebisacrylamide, cyclopentadienyl acrylate, and triallylcyanurate. Of particular importance in this context are, in addition,the methacrylic and acrylic acid C₁-C₈-hydroxyalkyl esters, such asn-hydroxyethyl, n-hydroxypropyl or n-hydroxybutyl acrylate andmethacrylate, and also compounds such as diacetoneacrylamide andacetylacetoxyethyl acrylate and methacrylate. In the case of aqueouspolymer dispersions produced exclusively by the method of free-radicalaqueous emulsion polymerization, the abovementioned monomers arecopolymerized customarily in amounts of from 0.5 to 10% by weight, basedon the total amount of monomers to be polymerized.

The preparation of aqueous polymer dispersions has been describedbeforehand on numerous occasions and is therefore sufficiently wellknown to the skilled worker [cf. e.g. Encyclopedia of Polymer Scienceand Engineering, Vol. 8, pp. 659 ff. (1987); D.C. Blackley, in HighPolymer Latices, Vol. 1, pp. 35 ff.(1966); Emulsion Polymerisation,Interscience Publishers, New York (1965) and Dispersionen synthetischerHochpolymerer, F. Hölscher, Springer-Verlag, Berlin (1969)]. It takesplace by emulsion polymerization of monomers having at least oneolefinically unsaturated group in the presence of a preferablywater-soluble polymerization initiator and of emulsifiers and, ifdesired, protective colloids and customary further additives. Ingeneral, the monomers in this case are added by continuous feed. Asinitiator it is preferred to use peroxodisulfuric acid and/or its saltsin amounts from 0.1 to 2% by weight based on the total amount of themonomers. The polymerization temperature is generally from 20 to 150° C.and, preferably from 60 to 120° C. The polymerization may take placeunder superatmospheric pressure. Emulsifiers used are, in particular,anionic emulsifiers alone or in a mixture with nonionic dispersants, inan amount of especially from 0.5 to 6% by weight of the total monomeramount.

The aftertreatment of the aqueous polymer dispersion that is carried outin accordance with the invention to reduce the amount of residualmonomer is conducted in particular after at least 95 and, preferably, atleast 98 to 99% by weight conversion of the total monomer amount in thefree-radical emulsion polymerization. The conditions in the case of themain polymerization and in the case of the aftertreatment are generallydifferent. Thus during the main polymerization, with a highconcentration of monomers and of growing and hence more and morehydrophobic oligomer radicals, the entry of radicals into the dispersionparticles takes place readily, whereas such entry is generally verydifficult during the aftertreatment, owing to the low monomerconcentration and the lack of growing oligomer radicals. In thepreparation of aqueous polymer dispersions, therefore, differentinitiator systems are generally required for the main polymerization andfor the aftertreatment.

Like processes of free-radical polymerization in general, the process ofthe invention, too, normally takes place under an inert gas atmosphere(e.g. N₂, Ar).

It is of course possible to subject the aftertreated aqueous polymerdispersions to stripping with inert gas and/or steam.

The free-radical redox initiator systems to be employed in accordancewith the invention permit an effective reduction in the amount ofresidual monomer within a relatively short time. It is also worth notingthat the reducing agent claimed by the invention is unable to reduce themicrobicides that are generally added as preservatives to aqueouspolymer dispersions, an advantage which means that the use of saidreducing agent in excess does not affect the quality of the aqueouspolymer dispersion in that respect.

EXAMPLES Example 1

A styrene-n-butyl acrylate dispersion prepared by free-radicalpolymerization from 23 kg of styrene, 25 kg of n-butyl acrylate, 2 kg ofacrylic acid, 1.2 kg of styrene seed latex dispersion (34% by weight inwater, particle size about 30 to 35 nm), 300 g of sodium lauryl sulfatesolution (15% strength by weight in water), 500 g of Dowfax® 2A1solution (dodecylphenoxybenzene-disulfonic acid sodium salt), 45%strength by weight in water), 300 g of sodium hydroxide solution (25%strength by weight in water), 150 g of sodium peroxodisulfate and 46 kgof water at 80° C. had a solids content of 52% by weight and a pH of4.3. The dispersion contained 8404 ppm of n-butyl acrylate, 1172 ppm ofstyrene and 2900 ppm of acrylic acid.

In the aftertreatment, 1350 g of this aqueous polymer dispersion,adjusted to a pH of 6.5 using 25% strength by weight aqueous sodiumhydroxide solution, were heated to 85° C. and then 0.02 g of thesodium/iron-EDTA complex was added. Subsequently, with stirring,

a) 30 g of a 1.2% strength by weight aqueous hydrogen peroxide solutionand

b) 30 g of a 2.35% strength by weight aqueous α,α′-dihydroxyacetonesolution

were metered in simultaneously in two separate feed streams each at arate of 30 g per hour. The resultant residual amounts of n-butylacrylate and styrene were determined by gas chromatography and theresidual amounts of acrylic acid by HPLC. The results obtained in theaftertreatment are set out in Table 1.

TABLE 1 Residual monomer amounts for the aqueous polymer dis- persion inthe aftertreatment n-butyl acryl- time styrene ate acrylic acid min ppmppm ppm 0 1172 8404 2900 60 170 4229 120 54 2438 240 46 2079 900

Example 2

A styrene-n-butyl acrylate dispersion prepared by free-radicalpolymerization from 23 kg of styrene, 25 kg of n-butyl acrylate, 2 kg ofacrylic acid, 1.2 kg of styrene seed latex dispersion (34% by weight inwater, particle size about 30 to 35 nm), 300 g of sodium lauryl sulfatesolution (15% strength by weight in water), 500 g of Dowfax® 2A1solution (dodecylphenoxybenzene-disulfonic acid sodium salt), 45%strength by weight in water), 300 g of sodium hydroxide solution (25%strength by weight in water), 150 g of sodium peroxodisulfate and 46 kgof water at 80° C. had a solids content of 52% by weight and a pH of4.3.

52 g of distilled water were added to 1298 g of this dispersion to givea dispersion having a solids content of 50% by weight. The dispersiondiluted to 50% by weight and adjusted to a pH of 7.0 using 25% strengthby weight aqueous sodium hydroxide solution contained 5785 ppm ofn-butyl acrylate, 393 ppm of styrene and 1800 ppm of acrylic acid. Inthe aftertreatment, this aqueous polymer dispersion was heated to 85° C.and then 0.034 g of the sodium/iron-EDTA complex was added.Subsequently, with stirring,

a) 30 g of a 11.3% strength by weight aqueous hydrogen peroxide solutionand

b) 30 g of a 11.3% strength by weight aqueous α-hydroxyacetone solution

were metered in simultaneously in two separate feed streams each at arate of 10 g per hour. The resultant residual amounts of n-butylacrylate and styrene were determined by gas chromatography and theresidual amounts of acrylic acid by HPLC. The results obtained in theaftertreatment are set out in Table 2.

TABLE 2 Residual monomer amounts for the aqueous polymer dis- persion inthe aftertreatment n-butyl acryl- time styrene ate acrylic acid min ppmppm ppm 0 393 5785 1800 60 163 4094 120 42 1771 240 36 116 90

Example 3

A styrene-butadiene dispersion was prepared by free-radicalpolymerization from 6 kg of styrene, 8.4 kg of 1,3-butadiene, 3.6 kg ofacrylic acid, 0.41 kg of styrene seed latex dispersion (34% by weight inwater, particle size about 30 to 35 nm), 135 g of Texapon® NSO solution(sodium lauryl ether sulfate having on average 2.5 ethylene oxide units;28% strength by weight in water), 150 g of sodium hydroxide solution(10% strength by weight in water), 110 g of sodium peroxodisulfate and18.28 kg of water at 82° C. Excess 1,3-butadiene was removed byinjecting 2 bar of nitrogen with stirring into the dispersion, which hadbeen cooled to 68° C., releasing the pressure and applying a slightunderpressure (750 mbar absolute) and repeating this procedure inautomated form about 1500 times. After 4 hours, a dispersion wasobtained which had a solids content of 51.3% by weight, a pH of 4.8, andresidual monomer contents of 1669 ppm of styrene and 280 ppm of acrylicacid.

In the aftertreatment, 1350 g of this aqueous polymer dispersion wereheated to 85° C. and then 0.02 g of the sodium/iron-EDTA complex wasadded. Subsequently, with stirring,

a) 30 g of a 1.2% strength by weight aqueous hydrogen peroxide solutionand

b) 30 g of a 2.3% strength by weight aqueous α,α′-dihydroxyacetonesolution

were metered in simultaneously in two separate feed streams each at arate of 10 g per hour. The resultant residual amounts of styrene weredetermined by gas chromatography and the residual amounts of acrylicacid by HPLC. The results obtained in the aftertreatment are set out inTable 3.

TABLE 3 Residual monomer amounts for the aqueous polymer dis- persion inthe aftertreatment time styrene acrylic acid min ppm ppm 0 1669 280 60906 120 545 180 383 20

We claim:
 1. A process for reducing the amount of residual monomer inaqueous polymer dispersions by aftertreatment with an initiator system,which comprises said aftertreatment in the aqueous polymer dispersionbeing accompanied by the addition of an initiator system comprising a)from 0.001 to 5% by weight, based on the total monomer amount used toprepare the polymer dispersion, a₁) of an oxidizing agent R¹OOH,  whereR¹ is hydrogen or a C₁-C₈ alkyl or a C₆-C₁₂-aryl group, and/or a₂) of acompound which in aqueous medium releases hydrogen peroxide, and b) from0.005 to 5% by weight, based on the total monomer amount used to preparethe polymer dispersion, b₁) of an α-hydroxyketone and/or anα-hydroxyaldehyde compound

 where R² and R³ independently of one another are hydrogen and/or aC₁-C₁₂-alkyl group which may contain functional groups and/or may beolefinically unsaturated, or R² and R³ optionally, by way of methylenegroups, form a ring structure which may contain functional groups and/ormay be olefinically unsaturated, and/or b₂) of a compound which inaqueous medium releases such α-hydroxyketone and/or α-hydroxyaldehydecompounds, and c) optionally, catalytic amounts of a polyvalent metalion which is able to exist in a plurality of valence states.
 2. Aprocess as claimed in claim 1, wherein the oxidizing agent is aninorganic compound.
 3. A process as claimed in claim 1, wherein theoxidizing agent is a hydrogen peroxide.
 4. A process as claimed in claim1, wherein R² is hydrogen and R³ is a methyl group.
 5. A process asclaimed in claim 1, wherein R² is hydrogen and R³ is a hydroxymethylgroup.
 6. A process as claimed in claim 1, wherein R² and R³ are each amethyl group.
 7. A process as claimed in claim 1, wherein component a)and component b) are supplied simultaneously in separate feed streams tothe polymer dispersion during the aftertreatment.
 8. A process asclaimed in claim 1, wherein the metal ions are added to the polymerdispersion in the aftertreatment prior to component a) and component b).9. A process as claimed in claim 1, wherein iron ions are employed aspolyvalent metal ion.
 10. A process as claimed in claim 1, wherein theiron ions are added in complexed form.
 11. A process as claimed in claim1, wherein the temperature of the polymer dispersion during theaftertreatment is from 50 to 130° C.
 12. A process as claimed in claim1, wherein the aftertreatment is conducted at superatmospheric pressure,at atmospheric pressure (1 bar absolute) or at subatmospheric pressure.13. A process as claimed in claim 1, wherein the pH of the polymerdispersion during the aftertreatment is ≧2 and ≦10.
 14. A process asclaimed in claim 1, wherein component b) is at least one compoundselected from the group consisting of glycol aldehyde,2,5-dihydroxy-1,4-dioxane, phenyl glycol aldehyde,2-hydroxy-3-phenylpropionaldehyde, glyceraldehyde, aldotetroses,aldopentoses, aldohexoses, α-hydroxyacetone, α,α′-dihydroxyacetone,1-hydroxy-2-butanone, 1-hydroxy-2-pentanone, 1-hydroxy-2-hexanone,3-hydroxy-2-butanone (acetoin), 4-hydroxy-2-hepten-5-one,2-hydroxy-3-pentanone, 3-hydroxy-2-pentanone, 3-hydroxy-4-heptanone,4-hydroxy-3-heptanone, 4-hydroxy-2,2-dimethyl-3-pentanone,3-hydroxy-2,2-dimethyl-4-pentanone, 2-hydroxy-1-phenyl-1-propanone,1-hydroxy-1-phenyl-2-propanone, 1-hydroxy-1-phenyl-2-butanone,2-hydroxy-1-phenyl-1-butanone, 2-hydroxy-1,2-diphenylethanone (benzoin),2-hydroxy-1-phenyl-1,4-pentanedione,1-hydroxy-1-phenyl-2,4-pentanedione, 2-hydroxycyclohexanone (adipoin),and 2-hydroxycyclopentanone (glutaroin).
 15. A process as claimed inclaim 1, wherein component b) is at least one compound selected from thegroup consisting of α-hydroxyacetone, α,α′-dihydroxyacetone,1-hydroxy-2-butanone, 1-hydroxy-2-pentanone and 3-hydroxy-2-butanone(acetoin).
 16. A process as claimed in claim 1, wherein component b) isat least one compound selected from the group consisting ofα-hydroxyacetone and α,α′-dihydroxyacetone.